1. Field of the Invention
This invention relates to a process for preparation of synthesis gas from black liquor gasifiers to be useful in a downstream synthesis. In particular, the invention concerns gasification of black liquor in a high temperature, downflow type gasifier to a synthesis gas stream with improved composition for use in the synthesis of valuable chemicals, especially DME.
2. Description of the Related Art
Synthesis gas can be prepared by gasification of so called black liquor (BL) by means of a blast i.e. oxygen, oxygen enriched air or air.
BL is an intermediate product stream from papermaking processes. It results from cooking of e.g. wood chips with aqueous sodium hydroxide and subsequent reduction in water through evaporation to a water content of about 70% by weight. The BL substance is rich in hemicellulose and lignin and has a relatively high content of inorganic material, forming large amounts of melts during the gasification process.
The known BL gasifier furnace types are constructed in different ways. To this end the gasifiers have either upflow or downflow of the gaseous reaction medium in the gasification chamber and are operated with more or less addition points of blast. In low-temperature BL gasifier types (˜600° C. gasification exit temperature) ash is withdrawn as a dry solid rather than a melt. In high-temperature BL gasifier types (˜900-1050° C. gasification exit temperature) the inorganic material originating from the BL leaves the gasifier as a liquid melt, which is quenched in a down-stream section.
In the high temperature downflow gasifier, the black liquor is normally supplied through burner means. In the gasification zone exothermic oxidation and endothermic disintegration reactions of the organic substances take place. The resulting temperature from the gasification process is typically about 900-1050° C.
The synthesis gas produced from the organic material dissolved in BL is rich in hydrogen, carbon monoxide and carbon dioxide. The synthesis gas contains additionally water, inert components depending on the composition of the black liquor and oxygen feed and sulphur components together with ash melt of Na2CO3 and NaS or equivalents of potassium salts.
In this type of gasifier the effluent from the gasification process is usually cooled by quenching with liquid water being sprayed into a quench chamber beneath the gasification chamber.
The melt is thus dissolved in the quench water resulting in formation of green liquor which after caustification is ready for dissolving organic material.
The synthesis gas may be purified and used either as fuel or as feed stock for downstream chemical processes.
A typical composition of the black liquor (BL) substance by weight C 36.14%, H 3.50%, O 34.3%, N 0.14%, S 4.80%, Cl 0.24%, Na 18.6% and K 2.02%. The dry mass percentage of the black liquor is about 70 wt %.
The composition of the synthesis gas produced depends on the BL feed composition, its dry mass percentage, the type of blast and BL atomizing streams and their relative flow rates. The synthesis gas composition depends furthermore on the gasifier operating conditions.
The gasifying process substantially disintegrates the organic mass by thermal cracking into e.g. CO, CH4 and by the homogeneous water gas shift reactionH2O+COH2+CO2  (1)in further amounts of H2 and CO2.
To this end, it is known that sodium compounds promote the water gas shift reaction (see f. inst. Sealock, L. J. Jr.; Elliott, D.C. in “Development of an advanced water-gas shift conversion system”, 4th annual advanced gasification contractors' meeting; 26 Jun. 1984; Morgantown, W. Va., USA).
Whereas the homogeneous shift reaction is slow at the temperatures prevailing in the quench section of the BL gasifier, the shift reaction due to its promotion by Na compounds may take place down to 250-300° C. Although it will be very slow below 400° C.
In the Lurgi Dry Ash Gasification process raw synthesis gas from a downstream process water cooler is recycled in a split stream to the gasifier for the provision of lock pressurizing gas in order to avoid dilution of the synthesis gas with inert gases.
In the Shell Coal Gasification Process and in the Prenflo Process a split stream of hot quench gas (about 250-280° C.) from a downstream slag filter is recycled to the gasifier to quench cool the effluent.
None of these processes result in changes of the composition of resulting raw synthesis gas.
As mentioned above the composition of the synthesis gas generated by the BL gasification depends e.g. on the gasifier feed compositions and operating conditions. Typically, the H2/CO ratio in the synthesis gas produced from BL gasification is in the range 0.85-1.5, varying over time as the BL feed will vary naturally e.g. with respect to atomic composition, energy content and rheology. The water content in the BL varies as a result of operating changes in the pulp manufacture. Also, there will be an upper limit on the practicable dry mass percentage of BL amongst other setting a limit to how low a ratio of H2/CO ratio can be obtained in the synthesis gas produced from the process.
Conventionally steam is used as the atomising stream in BL gasifiers, because it is readily available at the gasifier pressure (generated on site). An atomising stream is not strictly required, but has a great impact on the efficiency of the gasifier. A minimum flow rate of the atomising stream is about 0.05 kg/kg BL.
In the production of chemical compounds from synthesis gas it is usually necessary to adjust the composition of the gas in order to be useful in the production. Surplus of any reactant will result in additional requirements to the product separation. Especially, where the product is difficult to separate from unreacted components the separation will become prohibitively expensive rendering the process economically unviable.
Increasing interest has been shown to dimethyl ether (DME) over the last decades. DME is an environmentally benign chemical with a wide range of applications. DME can be used as a propellant, as a substitute for LPG in house hold apparatuses and as diesel fuel.
DME is produced from synthesis gas at a pressure typically in the range of 20-100 bar and at temperatures between 200 and 350° C. in one or more beds of catalyst/s promoting the following reactions:4H2+2CO2CH3OH  (2)H2O+COH2+CO2  (3)2CH3OHCH60+H2O  (4)3H2+3COCH6O+CO2  (2)+(3)+(4)
The appropriate H2/CO mole ratio in the synthesis gas for production of DME is about 1, preferably 0.9 to 1.25.
Downstream or integrated in the DME synthesis section, produced DME and by-product CO2 may be separated from the stream of unconverted synthesis gas, while a purge gas stream lean in CO and CO2, however, typically richer in H2 and inerts is sent to e.g. a fired heater or an auxiliary boiler as fuel.
With the high volatility of DME the efficiency of the synthesis process is sensitive to large variations of the H2/CO ratio in BL generated synthesis gas.
If the BL gasifier shall provide synthesis gas for use chemical processes, it will usually be necessary to adjust the gas composition independent of the pulp manufacture and the characteristics of the resulting BL to provide an optimal synthesis gas composition.
Thus, it is the general object of this invention to provide a process for the preparation of synthesis gas with an appropriate composition for use in the production of chemical compounds independent of the pulp manufacture and the BL characteristics.
Depending on the raw synthesis gas composition as it is produced in the BL gasifier, it will be necessary either to increase or decrease the H2/CO ratio in the raw gas to reach at the appropriate ratio.